This application describes an innovative approach to the rapid assessment of complex drug interactions between protease inhibitors and nonnucleoside reverse transcriptase inhibitors (NNRTIs), and commonly prescribed medications including methadone, ethinyl estradiol, fluconazole, pravastatin, and fluoxetine in HIV-infected, substance abusers The proposed methodology employs a Therapeutic Drug Monitoring (TDM) program that will facilitate rapid determination of ART in subjects receiving multiple interacting medications Specific aims 1) Implement a TDM program that will establish a mechanism to investigate protease inhibitor (PI) and NNRTI pharmacokinetics in HIV-infected, substance abusers receiving ART, determine drug exposure parameters (Cmin, AUC) and inhibitory quotients (IQs), 2) Determine the pharmacokinetics of selected interacting medications (methadone, ethinyl estradiol, fluconazole, pravastatin, fluoxetine)in HIV- infected, substance abusers receiving ART, 3) Determine in vitro and ex vivo total and unbound plasma concentrations of protease inhibitors and NNRTIs in HIV-infected, substance abusers utilizing novel analytical approaches including HPLC, LC-MS-MS and capillary electrophoresis, 4) Develop and validate a capillary electrophoresis assay capable of enantiomeric separation to enhance the pharmacokinetic analysis of interacting medications, 5) Examine pharmacogenetic factors that may identify individuals at greater risk for insufficient or excessive systemic drug exposure while receiving complex regimens with multiple drug- drug interactions The proposed TDM-drug-drug interaction program integrates a comprehensive antiretroviral clinical pharmacology research group, an HPLC/LC-MS analytical facility, a pharmacometrics laboratory, a web- based TDM enrollment infrastructure, and four HIV clinical centers caring for substance abusers Innovative pharmacokinetic and pharmacodynamic modeling approaches to assess complex drug-drug interaction analyses of PI and NNRTIs from HIV-infected substance abusers and non-substance abusers will be conducted Clinical sites will enroll subjects while drug measurement and data analysis will be conducted at the central pharmacology laboratory These studies will provide insight into clinical interactions that face clinicians and patients today, and identify priority drug interactions that require more traditional pharmacokinetic trials to identify specific mechanisms of interaction.